A glass fiber-reinforced polyamide resin is used in various industrial fields, taking advantage of its excellent mechanical strength. The glass fiber-reinforced polyamide resin is typically produced by a process involving kneading the polyamide resin with short fibers such as chopped strand using an extruder. However, this process has a problem that it cannot serve the need for a high level of mechanical characteristics because the glass fibers are shortened during the kneading in the extruder.
In response, it has recently been discussed to lengthen the reinforcement fiber in order to sufficiently bring out the native performance of a fibrous reinforcement material. A long glass fiber-reinforced polyamide resin can be obtained, for example, using a pultrusion process involving employing glass fiber rovings to impregnate a resin into the glass fiber rovings while taking off a strand. The polyamide resin composition obtained by this process is excellent in mechanical strength, as compared to the above-described short fiber-reinforced polyamide resin (for example, Patent Documents 1 and 2).
However, the above method requires considerably reducing the melt viscosity of the polyamide resin during production for sufficient impregnation of the resin into the glass fiber rovings. For that reason, it is necessary to set the melt temperature at a high level during production or use a polyamide resin having a low molecular weight. As a result, a problem is posed that long-term performances such as vibration fatigue resistance are not sufficient.
It is also necessary to slow the speed of taking off the resin strand in view of the impregnation of the resin into the glass fiber rovings. Thus, there is a problem that the polyamide resin has a longer melt residence time, which results in deteriorated color tone of the polyamide resin composition.
On the other hand, there is a production process involving impregnating a resin into a reinforcement fiber and taking off the reinforcement fiber while twisting to provide a strand (for example, Patent Documents 3, 4, and 5).
The above technique can stably produce, for a long period of time, a fiber-reinforced resin strand excellent in flexibility and buckling resistance by imparting twisting to the resin strand to impregnate the resin sufficiently into the reinforcement fiber.
However, these documents give no indication of a process for obtaining a long fiber-reinforced polyamide resin composition excellent in color tone, which is a challenge with a polyamide resin.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 46-004545 (corresponding U.S. Pat. No. 3,742,106)        Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-016463        Patent Document 3: Japanese Unexamined Patent Application Publication No. 06-254850        Patent Document 4: Japanese Unexamined Patent Application Publication No. 05-169445        Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-175512        